Such cylindrical filter cartridges are inserted in so-called filter housings, whereby the filter cartridge is inserted in a circular cartridge opening, e.g. in a bottom plate in the respective filter housing, and is secured in the cartridge opening by an upper collar on the filter cartridge. A multiple of these filter cartridges can be placed in a filter housing.
The filters are used e.g. for the cleaning of air, in that a suction apparatus sucks air through the filter and further into the filter housing.
Particles of dust and impurities settle as a layer on the outer side of the cylindrical filters, which at suitable intervals are cleaned e.g. by blowing air through the filter in the opposite direction or by shaking the filter housing. After some time, the filters get worn-out and must therefore be easy to replace.
This object forms the basis of patent publication U.S. Pat. No. 5,632,791, which discloses a substantially cylindrical, pleated filter cartridge which comprises a resilient top sleeve in which there is formed a peripheral groove, an inner, cylindrical, relatively stiff screen, an outer pleated filter cartridge and a resilient bottom element. The top sleeve is configured to fit into the circular cartridge opening in the bottom cap of the filter housing.
This type of filter cartridge is mounted inside the filter housing and, in the mounted condition, protrudes out of the housing.
In addition to the peripheral groove, the top sleeve of the filter cartridge comprises a collar which prevents the cartridge from moving through the actual cartridge opening in the filter housing. Moreover, a segment of the top sleeve below the peripheral groove is conical.
The top sleeve thus allows itself to be pressed down through the cartridge opening, and a seal is achieved against the edge of the opening, respectively at the groove, the collar and the upper edge of the conical segment.
The conical segment of the top sleeve naturally allows itself to be deformed under the influence of a certain force, in that the material is an elastomer, and in that the peripheral groove constitutes a weakening of the thickness of the top sleeve, whereby the collar at the groove can be bent like a hinge in relation to the conical segment.
The filter cartridge is inserted by applying a suitable force on the top of the collar, which can be effected e.g. by treading the filter cartridge in place in the cartridge opening.
On the other hand, the removal of the filter cartridge cannot be effected without the use of tools, which are typically pushed in under the collar, after which the sleeve is twisted loose, the reason being that the sleeve is typically subjected to a suitably high pre-stressing force in order for a sufficiently effective sealing to be achieved.
To further ensure the securing of this known filter cartridge, a permanent clamping band of e.g. stainless steel can be inserted at the inner side of the sleeve opposite the peripheral groove, whereby the sleeve opposite the groove is reinforced.
According to patent publication U.S. Pat. No. 5,632,791, it is suggested to use a band which has a diameter which is slightly greater than the inside diameter of the sleeve, and which before being inserted is twisted into a figure eight shape and then inserted into the inner side of the sleeve opposite the peripheral groove. The band increases the pressure against the edge of the cartridge opening, whereby a more effective seal is achieved. However, it is a disadvantage with the above-mentioned type of filter, especially with long filters, that vibrations can arise in the filter, whereby the groove is exposed to frictional influences and herewith wear, which can result in the filter becoming defect.
In FIGS. 1 and 2 there is shown a filter cartridge 1 with a sleeve 22 and a bottom 21, and a filter 3 of the same type as the cartridge according to this patent publication.
Another type of filter cartridge is known from patent publication WO 90/11116, which is inserted on the outer side of the filter housing and which consists mainly of the following two parts which are of the same material, preferably polyurethane: An outer filter part consisting of a top sleeve, a bottom sleeve and a cylindrical, pleated filter which is placed between them, and an inner filter part, similarly with a top and bottom part and a cylindrical filter which at the same time serves as a strengthener.
The outer part of the top sleeve has an outer groove which engages with the cartridge opening for the filter, and in which the filter hangs.
Diametrically opposite the outer groove on the outer part of the top sleeve, the inside of the top sleeve is provided with a further groove. The inner part has a projection or a ridge which enters into engagement with this groove, whereby the securing of the outer part to the cartridge opening is stabilised, and whereby the correct axial positioning of the inner part is determined.
When mounting the filter, which is intended to be mounted from the outer side of the filter housing, the outer part is first secured in the cartridge opening at the above-mentioned outer groove. Hereafter, the inner part including the inner filter is inserted in the outer part until the above-mentioned projection engages in the internal groove. Moreover, a projection and a groove are also provided in the filter's bottom parts.
Removal of the filter is normally effected by inserting a knife-like tool in between the cartridge opening and the groove in the top sleeve, after which the filter in assembled condition can be twisted loose. However, this typically requires the use of a considerable amount of physical effort.
Finally, from the U.S. Pat. No. 5,308,485, there is known a filter cartridge with an expansion part. The expansion part comprises an annular, radially-extending bead arranged to deform the filter's collar so that it is secured in the cartridge opening. This gives rise to several disadvantages, the most significant of which is that the collar of the filter, the expansion part and the cartridge opening must fit one another quite precisely from the point of view of dimensions. If this is not the case, use must be made of a spacing element in order for the filter to be mounted correctly, as explained and shown in FIG. 8 of the American patent publication. Such demands that the individual parts must fit each other precisely made it necessary for stocks to be held of filters, expansion parts and spacing elements in many dimensions in order to be able to supply filters for filter housings of different types. Furthermore, problems arise regarding different plate thicknesses in the cell plates provided with openings in the filter housings of different constructions and embodiments. Finally, it can be ascertained that the construction according to the latter-mentioned American patent consists of four parts, which increases the costs.